1. Field of the Invention
This invention relates to a process for bending a tubular piece of work to form a bent metal tube having a small radius of curvature, and to a bent metal tube of high quality made preferably-by that process, and having no flattening, crack, break or other defect of the nature formed in its bent portion by the concentration of stress in the tube.
2. Description of the Prior Art
The increasing use of small parts for the engine room, or cooler mechanism of an automobile, etc. has brought about a demand for bent metal tubes having a small radius of curvature for connecting the parts arranged with a high packing density. A bent metal tube having a relatively large radius of curvature can be manufactured if work is clamped at one end, and pressed against a bending die. If this manufacturing process is used for bending a tubular piece of work to a radius of curvature which is smaller than its diameter, however, it has been likely that the work may slip back at its clamped end during the rotation of the bending die, and form a bent metal tube flattened by the failure of its bent portion to maintain its circular cross section, or a bent tube having a folded or collapsed wall portion along the inner center of its curvature, or a broken wall portion along the outer center thereof.
FIG. 11 shows examples of defectively bent metal tubes: (a) one having a flattened wall portion .alpha. along the outer corner of its bend, (b) one having a cracked wall portion .beta. along the outer corner of its bend, and (c) one having a collapsed wall portion .gamma. along the inner corner of its bend. While the tube having the cracked wall portion is, of course, undesirable as it allows a fluid to leak out, the tube having the flattened or collapsed wall portion .alpha. or .gamma. is also undesirable, since it gives a greater resistance to the flow of a fluid therethrough, and the concentration of stress in the collapsed wall portion is likely to cause it to crack.
It has, therefore, been usual to manufacture a bent metal tube or pipe by cutting one end of each of two pipes at an angle of 45.degree., and welding the cut ends thereof together. The manufacture of a bent tube in this way has, however, had the drawbacks of calling for a long time because of a series of steps including oblique cutting, welding and leakage testing, and of yielding a bent tube which is likely to give resistance and a pressure loss to a fluid flowing through its right-angled joint, as well as allowing the fluid to leak out through its welded joint.